1. Field of the Invention
This invention relates to an image processing apparatus and a controlling method therefor, and for example, to an image processing apparatus for forming an image by the electrophotographic process and a controlling method therefor.
2. Description of the Related Art
FIGS. 1 and 2 of the accompanying drawings are typical views showing the printing by a color image processing apparatus for successively transferring yellow, magenta, cyan and black colors to a recording material conveyed by a conveying belt.
When recording sheets long in a conveying direction are to be printed in the order of monochrome and full color, image formation in respective stations for yellow, magenta and cyan is not effected on the preceding recording sheet (shown as the first page in FIG. 1) to be monochromatically printed. At timing whereat the leading end of the recording sheet has arrived at a position corresponding to the exposure starting position of a black photosensitive drum, a vertical synchronization signal (hereinafter expressed as the “TOP signal”) indicative of the output timing of black image data is outputted to a controller, not shown.
Next, a timing whereat the leading end of the succeeding recording sheet (shown as the second page in FIG. 1) to be color-printed has arrived at a position corresponding to the exposure starting position of a yellow photosensitive drum, a TOP signal indicative of the output timing of yellow image data is outputted to the controller.
However, in the image processing apparatus shown in FIGS. 1 and 2, in order to decrease the number of signal lines, the TOP signals uses a single signal line (a single signal) irrespective of the monochromatic print and the full color print.
FIG. 2 shows a state in which in the image processing apparatus of the same construction as that shown in FIG. 1, recording sheets short in the conveying direction are printed in the order of monochromatic color and full color.
When as shown in FIG. 2, the distance between the position of the yellow photosensitive drum and the position of the black photosensitive drum is defined as L1 and the length of the recording sheet in the conveying direction is defined as L2 and the distance between that recording sheet and the succeeding recording sheet is defined as L3, when the relation among them is L1>L2+L3, the image formation of (the first color yellow in) the succeeding full color print precedes the preceding monochromatic print. So, the sheet interval L3 is widened so that the order of image processing may not be reversed.
FIG. 3 of the accompanying drawings is a timing chart in continuous printing wherein full color and monochrome are mixedly present, and shows an example in which printing is effected in the order of monochrome, monochrome, full color, monochrome, full color and full color.
The TOP signal is used in both of full color print and monochromatic print, and what are designated by the reference numeral 301 in FIG. 3 are the TOP signals for full color print, and what are denoted by the reference numeral 401 are the TOP signals for monochromatic print.
A controller, in monochromatic print, outputs black image data 403 in synchronism with the TOP signals 401, and in full color print, outputs first color, i.e., yellow image data 303 in synchronism with the TOP signals 301, and thereafter outputs magenta, cyan and black image data (designated by the reference numerals 305, 307 and 309, respectively, in FIG. 3). Times denoted by T1, T2 and T3 in FIG. 3 correspond to the times for which a recording sheet is conveyed over distances from the yellow photosensitive drum to the other color photosensitive drums.
The number of printed sheets per unit time, i.e., the printing capability, in such a construction can be judged from the output timing of the black image data in both of full color and monochrome, as shown in FIG. 3.
When continuous print in which full color print and monochromatic print are mixedly present is to be effected, as is apparent from FIG. 3, the sheet interval L3 is greatly widened to avoid the above-noted problem when changeover takes place from monochromatic print to full color print, whereby the printing capability is reduced.